1. Field of the Invention
This invention relates to a rotary atomizing head type coating system, particularly suitable for use, for example, in coating operations on vehicle bodies or the like which require changes of paint colors.
2. Discussion of the Background
Generally, rotary atomizing head type coating systems have been in wide use for coating vehicle bodies and the like. In connection with coating operations of this sort, recent demands are mostly focused on measures for reducing the amounts of paint and solvent which have to be discarded as waste each time when changing the paint color as well as on measures for coping with as many color changes as possible in the course of a coating operation.
In this regard, Japanese Laid-Open Patent Publication No. H8-229446 discloses a rotary atomizing head type coating system which is provided with measures for reducing the amounts of waste paint and solvent, and which can cope with an increased number of paint colors. This rotary atomizing head type coating system employs paint cartridges of different colors which can be replaceably set in position within a housing together with a coating machine unit.
Further, the prior art rotary atomizing head type coating system is generally composed of a housing which is provided with a coating machine mount portion and a cartridge mount portion in its front and rear portions, respectively, and a coating machine which is mounted in the coating machine mount portion, including an air motor with a rotational shaft and a rotary atomizing head which is mounted on a fore end portion of the rotational shaft of the air motor. Provided internally of and axially through the rotational shaft of the air motor, which constitutes the coating machine, is a feed tube passage having its fore and rear ends opened into the rotary atomizing head and the cartridge mount portion, respectively.
The coating system is equipped with paint cartridges of different colors which are selectively and replaceably mounted on the cartridge mount portion of the housing. Each one of the paint cartridges is constituted by a cylinder which is filled with paint and a feed tube which is extended axially forward from the fore distal end of the cartridge cylinder. The cartridge cylinder is replaceably set in position within the cartridge mount portion on the housing, while the feed tube is passed through the above-mentioned feed tube passage.
The paint cartridge is further provided with a movable partition wall defining within the cartridge cylinder a paint reservoir chamber in communication with the feed tube and a paint-extruding air chamber, and an air passage which supplies paint-extruding air to the air chamber. In addition, to and in communication with the air passage on the side of the cartridge, an air passage is also provided on the side of the housing. Through the air passage on the side of the housing and the air passage on the side of the cartridge, paint-extruding air is circulated to the paint-extruding air chamber to displace the movable partition wall forward, thereby pushing the paint in the paint reservoir chamber into the rotary atomizing head through the feed tube.
In the case of the prior art rotary atomizing head type coating system which is arranged in the above-described manner, a paint cartridge of a particular color, which is selected from a number of cartridges of different colors, is mounted on the cartridge mount portion in the housing. Then, air is supplied to the paint-extruding air chamber of the cartridge under suitable pressure to push forward the paint in the paint reservoir chamber of the paint cartridge into the feed tube for supplying to the rotary atomizing head, which sprays the supplied paint over an article to be coated.
The paint color can be changed simply by replacing the cartridge on the machine by other paint cartridge of a different color, without necessitating to discard waste paint and solvent.
During a coating operation, the rotary atomizing head type coating system according to the above-described prior art is put in reciprocating movements and at the same time its position is successively shifted in a direction perpendicular to the direction of reciprocating movements. At this time, in order to maintain a uniform thickness of the coating film, the movement of the coating system is reversed at positions outside a coating surface of work. Besides, as the coating system comes to a position outside a coating surface, the supply of air to an air chamber of the paint cartridge is cut off to break the paint supply to the rotary atomizing head, thereby temporarily stopping paint spraying from the atomizer head.
However, even if the air supply is cut off to stop paint spraying, it is often the case that the air passages and air chamber remain in a pressurized state for some time after the air supply is cut off. Therefore, during that time period, the movable partition wall is displaced to continue the paint supply to the rotary atomizing head until the air passages and air chamber as well as the paint reservoir chamber and the feed tube settle down to a static pressure level.
Accordingly, after dispatching a command signal to stop the paint supply, there is always a time lag until the paint supply actually comes to a stop. This gives rise to various problems, for instance, such as wasteful paint consumption, paint depositions on unnecessary portions of coating surfaces, and irregularities in thickness of a coated film which would invite degradations in quality of the coating.
Further, when a paint cartridge is extracted from the housing for replacement, the residual paint which drips from the feed tube may fall on a coated surface to become a cause of coating defects.
Furthermore, the prior art coating system employs paint-extruding air for displacing the movable partition wall in the cartridge cylinder. However, a compressed fluid like air is apt to undergo variations in volume while being supplied to the extruding air chamber. Therefore, it is usually required to provide a flowmeter and various control valves additionally on the coating system in order to ensure accurate paint supply at the time of a coating operation, resulting in increases in size and running cost of the coating system.